1. Field of the Invention
The present invention relates generally to electronic stimulation devices to control the beating of hearts, especially hearts with pathologies that interfere with normal rhythmicity, electrical conduction, and/or contractility. In particular, the present invention relates to pacemakers used to overcome atrial fibrillation by use of 1) atrial sensing; 2) electrical test stimulation of the atria; and 3) multiple site stimulation in which the various atrial areas are slowly entrained to a common beating rate to produce electrical/functional conformity, i.e., cardioversion, with each case either eventuating in spontaneous reversion to a normal atrial rhythm, or reduced energy requirement for reversion by electrical countershock.
2. Background Information
Morbidity associated with malfunctions of the atria, while not immediate, is high. Atrial malfunctions of rhythmicity (e.g., atrial fibrillation, various atrial arrhythmias, A-V block and other conduction abnormalities, etc.) can contribute to thrombosis, emboli, stroke and/or heart failure, each of which can place a patient in significant peril.
Atrial Sensing A variety of approaches have been developed which use pacemakers to counter atrial malfunctions of rhythmicity, as well as attendant effects on ventricular function. In addition, sophisticated approaches have been developed for pacemaker systems to determine the nature of any particular ventricular malfunction, and whether a malfunction originates in the atria or in the ventricles. One such approach uses ventricular sensing to measure/determine the probability density function (pdf) on a moment-to-moment basis. For example, U.S. Pat. No. 5,163,429 to Cohen discloses the use of narrow window pdf data as but one criterion among several for assessing ventricular cardiac function. The use of pdf data to determine ventricular fibrillation also is disclosed in Implantable Cardioverter-Defibrillators (N. A. Estes III, A. Manolis and P. Wang, ed.). U.S. Pat. No. 5,421,830 to Epstein et al. (discussed further below) also discloses the use of pdf data as one set among a variety of data types that collectively are also used to assess cardiac function. The use of probability density function data for assessing atrial cardiac function has not been disclosed and presents its own unique difficulties as will be further discussed.
Electrical Test Stimulation of Atria. In a few limited cases, pacemaker protocols have been employed in which electrical test stimuli are applied to the atria, and the physiological responses thereto are monitored to aid in the determination of the best or most appropriate protocol to initiate, cure, or ameliorate the existing cardiac malfunction. For example, U.S. Pat. No. 5,620,471 to Duncan discloses three basic protocols for determining whether observed ventricular irregularities are actually caused by atrial arrhythmias. One protocol includes atrial electrical test stimulation, and all three protocols monitor both atrial and ventricular rhythms for three parameters: rates of atrial and ventricular firing, stability of firing/beating in atria and ventricles, and whether or not ventricular firing tracks atrial firing. In the first protocol, when the ventricular firing rate is less than the atrial firing rate (indicating no ventricular tracking of atrial beats), and firing rates are stable, then ventricular tachycardia is presumed, and ventricular stimulation is applied. On the other hand (second protocol), if the ventricular firing rate is not stable, then atrial arrhythmia is pressured, and atrial stimulation is applied. The third protocol is based on the fact that, when the ventricular firing rate equals the atrial firing rate, there may or may not be ventricular tracking of atrial firing. Whether or not there is ventricular tracking is determined by the presence or not of ventricular tracking following premature atrial stimulation by the pacemaker. If there is ventricular tracking of atrial firing, the arrhythmic mechanism is presumed to be atrial tachycardia. However, if there is no ventricular tracking of atrial firing, then ventricular tachycardia is presumed, and ventricular stimulation is performed.
U.S. Pat. No. 5,421,830 to Epstein et al. discloses a general means for recording, testing, and analyzing cardiac function based on data fromxe2x80x94and electrical test stimulation viaxe2x80x94a patient""s pacemaker, as well as data from additional sensors detecting hemodynamic or other body functions. Total intracardiac electrograms (reflecting both atrial and ventricular functional status) or just selected data (e.g., Pxe2x80x94P or Rxe2x80x94R intervals, heart rate, arrhythmia duration, slew rate, probability density function, etc.) may be recorded and analyzed. The patient""s atrial and ventricular responses to electrical test pulses may also be recorded. In sum, this system provides a means to more easily tailor settings for pacemakers to achieve optimal settings for the specific patient or for the specific situation (e.g., during exercise or exertion) of a given patient.
U.S. Pat. No. 5,215,083 to Drane et al. also discloses the use of electrical test stimulation to aid in the fine tuning and evaluation of different possible stimulation protocols for a patient""s heart. In particular, electrical test pulses are employed to induce ventricular fibrillation or tachycardia for use in evaluating the effectiveness of alternative programmed therapies.
Multiple Site Atrial Stimulation. The use of multiple site atrial stimulation has been disclosed for various purposes, such as defibrillation, cardioversion, pacing, and dc field production. One example is provided by U.S. Pat. No. 5,562,708 to Combs et al., which discloses the employment of large surface electrodes (each effectively comprising multiple electrodes) that are implanted to one or both atria for providing extended, low energy electrical impulses. The electrical impulses are applied simultaneously at multiple sites over atrial surfaces, and atrial fibrillation is interrupted by gradually entraining greater portions of atrial tissue. These pacemaker electrodes may be used for various purposes in addition to pacing, such as conventional defibrillation and cardioversion.
U.S. Pat. No. 5,649,966 to Noren et al. discloses the use of multiple electrodes for the purpose of applying a subthreshold dc field to overcome fibrillation. The rate of application of the dc field is sufficiently low so that no action potential is triggered. Polarity may also be changed periodically. In one embodiment, four electrodes are positioned within a single plane in the heart, which permits a dipole field in virtually any direction within that plane.
U.S. Pat. No. 5,411,547 to Causey, III discloses the use of sets of complex mesh patch electrodes, in which each electrode comprises an anode patch and a cathode patch, for purposes of cardioversion-defibrillation. Bidirectional cardiac shocking is permitted by these electrodes.
U.S. Pat. No. 5,391,185 to Kroll discloses the use of multiple electrodes to effect atrial defibrillation. The possibility of inducing ventricular fibrillation during the course of atrial defibrillation is greatly reduced by synchronizing the atrial stimulation to fall within the QRS phase of the ventricular cycle.
U.S. Pat. No. 5,181,511 to Nickolls et al. discloses the use of multiple electrodes in antitachycardia pacing therapy. The electrodes not only each serve an electrical sensing role (to locate the site of an ectopic focus), but also function in concert to create a virtual electrode for stimulating at the site of an ectopic focus.
Existing Needs. In the area of atrial malfunctions of rhythmicity what is needed is a means to entrain multiple atrial sites, but also in combination with an atrial sensing/ measurement capability that is coupled with atrial test stimulation and analysis capability. Atrial test stimulation and analysis capability is needed to provide better determination of the nature of the malfunction and the most probable or efficacious corrective therapy to undertake. Furthermore, the use of atrial test stimulation is critically needed for the fundamental reason that the physician cannot know a priori how a given heart (or a given heart under a particular medical or pathological condition) will respond to a selected stimulation regime, even if that selected stimulation regime would work generally for other cardiac patients. Thus, a trial-and-error testing capability needs to be available for pacemakers whose traditional stimulation regimes do not work for the occasional refractory patient. The multiple site stimulation capability is needed in order to more quickly and efficiently cardioconvert the atria in the face of arrhythmia, fibrillation, etc. Atrial sensing and use of measurement data are needed to better provide the physician and/or the circuit logic of the pacemaker with information as to the physiological state of the heart; i.e., whether there is atrial arrhythmia or fibrillation, where an ectopic focus is located, etc. Thus, what is needed is a pacemaker that combines all three of these elements: atrial sensing and measurement capability, atrial electrical test stimulation and analysis capability, and multiple site stimulation capability.
Lastly, a need also exists for a stimulation protocol which can travel more quickly across the myocardium and which provides improved cardiac entrainment along with the ability to entrain portions of the heart from a greater distance.
It therefore is an object of the present invention to provide a pacemaker that is capable of pacing atria from multiple sites.
It is another object of the present invention to provide a pacemaker that is capable of slowly entraining atria by stimulating the atria at multiple sites to produce electrical and functional conformity of the atria, with resulting increased pumping efficiency of the heart.
It is yet another object of the present invention to provide a pacemaker that is capable of detecting the presence of atrial fibrillation and atrial arrhythmias by stimulating the atria and observing and measuring the consequent effects on atrial and ventricular function.
It is a further object of the present invention to provide a pacemaker that is capable of obtaining and analyzing probability density function data from atria in order to determine atrial rates of beating and to assess atrial physiological function.
It is a further object of the present invention to provide an electronic stimulation device, for stimulating the atria from multiple sites, where the electrodes of the electronic stimulation device can be inserted intravenously.
It is a further object of the present invention to provide an electronic stimulation device, for stimulating the atria from multiple sites, where each electrode of the device has an independent generator.
It is a further object of the present invention to provide an electronic stimulation device for stimulating the atria from multiple sites, where each site is entrained separately and quickly brought to the same phase.
It is a further object of the present invention to provide an electronic stimulation device for stimulating the atria from multiple sites, to sequence the sites to mimic a normal heart beat.
It is a further object of the present invention to determine cardiac capture by monitoring cardiac activity and noting when the baseline of such activity is off zero.
It is a further object of the present invention to decrease threshold rises due to a build up of fibrous tissue.
The present invention accomplishes the above objectives by providing a cardiac pacemaker with a unique constellation of features and capabilities. In particular, a means for entraining multiple atrial sites is provided by the use of multiple electrodes. The multiple electrodes not only permit multi-site stimulation capability, but also multi-site sensing (including pdf measurement) capability, which, by triangulation, essentially provides the ability to determine the site(s) of any atrial ectopic focus. The multi-site stimulation capability inherently provides a system poised for more efficient entrainment and/or cardioconversion of the atria in the face of arrhythmia, fibrillation, etc. Combined with this multi-site stimulation/sensing capability is the means to execute trial-and-error testing and analysis to determine the best general stimulation protocol, to fine tune a given protocol, or to adjust a protocol in response to changes in the physiological/pathological status of the patient in general and/or the patient""s heart in particular.
Incorporating the use of biphasic stimulation with the present invention provides the additional benefits of reducing cardiac inflammation damage, reducing or eliminating threshold rises due to the buildup of fibrous tissue and extending battery life of the electrodes.
In addition, the ability to conduct trial-and-error testing, including the analysis of the data derived therefrom, permits more thorough and more definitive determination of the physiological status of the heart; this determination can practically approach a moment-to-moment basis when analysis is automated by appropriate software for the purpose.
In sum, the present invention provides a cardiac pacemaker that has greater functional capabilities for the patient""s atria than current technologies allow. The greater atrial xe2x80x9ccoveragexe2x80x9d from the strategic placement of multiple electrodes permits faster correction of atrial arrhythmia, fibrillation, etc. Similarly, the use of multi-site electrodes permits more accurate sensing, including the capability of locating the site(s) of any atrial ectopic focus so as to better apply corrective stimulation procedures. In addition, the ability to apply trial-and-error testing/analytical procedures permits quicker analysis and correction of malfunctions of electrical conduction, cardiac contractility, rhythmicity, etc. Thus, the present invention constitutes an advance in cardiac care procedures as they relate to atrial pacemakers. The end result for the patient is better treatment, and, hence, a better prognosis from the better and faster treatment.
The method and apparatus relating to biphasic pacing comprises a first and second stimulation phase, with each stimulation phase having a polarity, amplitude, shape, and duration. In a preferred embodiment, the first and second phases have differing polarities. In one alternative embodiment, the two phases are of differing amplitude. In a second alternative embodiment, the two phases are of differing duration. In a third alternative embodiment, the first phase is in a chopped wave form. In a fourth alternative embodiment, the amplitude of the first phase is ramped. In a fifth alternative embodiment the first phase is administered over 200 milliseconds after completion of a cardiac beating/pumping cycle. In a preferred alternative embodiment, the first phase of stimulation is an a nodal pulse at maximum subthreshold amplitude for a long duration, and the second phase of stimulation is a cathodal pulse of short duration and high amplitude. It is noted that the aforementioned alternative embodiments can be combined in differing fashions. It is also noted that these alternative embodiments are intended to be presented by way of example only, and are not limiting.
Enhanced myocardial function is obtained through the biphasic stimulation of the present invention. The combination of cathodal with anodal pulses of either a stimulating or conditioning nature, preserves the improved conduction and contractility of anodal stimulation while eliminating the drawback of increased stimulation threshold. The result is a depolarization wave of increased propagation speed. This increased propagation speed results in increased synchronization and reduced heterogenicity of myocardial depolarization resulting in superior blood flow and contraction. Improved stimulation at a lower voltage level also results in: 1/reduction in scar tissue buildup thereby reducing the tendency of the capture threshold to rise; 2/reduction in power consumption leading to increased life for pacemaker batteries; and 3/decreased potential for patient discomfort due to stimulation of the phrenic or diaphragmatic plexus or due to intercostal muscle pacing.